This invention relates generally to improved screwdrivers. More particularly, but not by way of limitation, this invention relates to an improved screwdriver driven by a pneumatic motor and including a torque-actuated shut-off mechanism and automatic reset.
Screwdrivers of the type described hereinafter are generally utilized in production work. In such work, the screwdrivers are preset to drive a threaded fastener into a workpiece up to a predetermined torque. Various arrangements have been proposed in the past to provide accurate and repetitive shut-off mechanisms in such screwdrivers.
U.S. Pat. Nos. 3,766,990, issued Oct. 23, 1973, to Richard E. Eckman et al entitled "Low Torque Automatic Screwdriver" and 4,154,308, issued May 15, 1979, to Emory G. Goldsbury et al and also entitled "Low Torque Automatic Screwdriver," propose one type of such shut-off and reset mechanism that has been successfully operated.
U.S. Pat. No. 3,811,513, issued May 21, 1974, to Hans Wezel et al and entitled "Portable Pneumatic Power Tool," proposes a different type of automatic shut-off mechanism. In the mechanism proposed by Wezel, a pair of balls are constrained within a drive shaft by a sleeve that encircles the drive shaft. The balls engage a valve actuating rod to prevent the rod from moving toward the tool bit and preventing the valve from closing until a predetermined torque has been reached. When this occurs, the sleeve is moved to a position wherein the balls can separate slightly, permitting the valve rod to move toward the tool bit and permitting the valve to close. While this tool apparently operates satisfactorily, it is believed that a more positive shut-off mechanism and reset therefor will greatly enhance the tool.
Therefore, an object of this invention is to provide an improved screwdriver including a positive shut-off and reset mechanism that is accurate and repeatable and one in which the shut-off mechanism can be quickly and easily reset to permit additional operation of the screwdriver.